Semiconductor device comprising buried channel region

ABSTRACT

A semiconductor device includes a gate insulating film formed on a semiconductor substrate between first diffusion layers, a gate electrode including a first gate portion formed on the gate insulating film and a second gate portion formed on the first gate portion, a first width in a channel direction of the first gate portion being substantially equal to a width in that of the gate insulating film, and a second width in the channel direction of the second gate portion being larger than the first width, a gate side wall insulating film including a first side wall portion formed on a side surface of the first gate portion and the gate insulating film and a second side wall portion formed on a side surface of the second gate portion, and a second diffusion layer formed apart from the first diffusion layers below the gate insulating film.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromthe prior Japanese Patent Application No. 2001-095899, filed Mar. 29,2001, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device and a method formanufacturing the same, particularly, to a gate electrode structure of adamascene gate transistor prepared by forming a gate electrode groove inan insulating film, followed by burying a gate electrode in the grooveand a method for manufacturing the particular gate electrode structure.

2. Description of the Related Art

A transistor using a metallic material for forming the gate electrode isconsidered hopeful as a next-generation transistor because this type oftransistor is free from deteriorations caused by, for example, a voltagedrop derived from the depletion of the gate or the gate resistance, likea transistor using polycrystalline silicon (polysilicon) for forming thegate electrode. However, a metallic material such as W, Al or Cu isincapable of withstanding a high temperature heat treatment. Therefore,the activation temperature for forming the source-drain regions afterthe processing of the gate electrode should not affect the gateelectrode, although the gate electrode is affected by the activationtemperature noted above in usual transistor forming processes. As ameans for preventing the gate electrode from being affected by theactivation temperature in question, a damascene metal gate orreplacement gate transistor has attracted attention as described in, forexample, “International Electron Devices Meeting Technical Digest, 1998,pp. 777-780, pp. 785-788”. The damascene gate or replacement gatetransistor is prepared as follows. In the first step, a dummy gate isformed, followed by implanting impurity ions with the dummy gate used asa mask. Then, the implanted impurity is activated so as to formsource-drain regions. Further, after removal of the dummy gate, ionimplantation is performed for forming a channel region, followed byforming again a gate insulating film and subsequently burying a metalelectrode.

However, in the case of using a metallic material for forming a gateelectrode, it is difficult to adjust the work function by ionimplantation, although ion implantation is employed for adjusting thework function in the case of using polysilicon for forming the gateelectrode. Therefore, it is unavoidable to select a material having awork function positioned in the central region of the band gap ofsilicon, such as TiN, for forming the gate electrode for a CMOStransistor, with the result that it is difficult to realize a lowvoltage/low threshold value required for a next-generation transistor ofreduced the power consumption.

Under the circumstances, in order to lower the threshold value, it isnecessary to employ a buried channel structure in which an impurityhaving a conductivity type opposite that of the semiconductor substrateis implanted in the channel surface.

However, an impurity having a conductivity type opposite that of theimpurity used for the adjustment of the threshold value of a surfacechannel type transistor is implanted in the buried channel typetransistor, with the result that a punch-through tends to take placebetween the source and drain regions, which deteriorates the shortchannel characteristics.

On the other hand, proposed is a so-called “pocket” or “halo” structurein which an impurity for the punch-through stopper is obliquelyintroduced by ion implantation after formation of the dummy gate, asshown in FIG. 19A, for improving the short channel characteristics, asdescribed in, for example, “International Electron Devices MeetingTechnical Digest, 1998, pp. 789-792”.

In the case of using this type of transistor, the impurity concentrationis rendered high in the channel portion if the channel is long, comparedwith a short channel, as shown in FIGS. 19B and 19C, so as to make itpossible to sufficiently suppress the punch-through between the sourceand drain regions.

In this particular structure, however, a junction is formed between thesource-drain regions having a high impurity concentration and the regionof a high impurity concentration in the halo structure, giving rise tothe problems that the junction capacitance between the source-drainregions and the semiconductor substrate, and that the junction leakcurrent is increased. Also, as described previously, a transistor whichwith a metal gate is considered hopeful as a next generation transistor.However, the activation temperature for forming the source-drain regionsafter the processing of the gate electrode should not affect the gateelectrode, although the gate electrode is affected by the activationtemperature noted above in the ordinary process for forming atransistor. As a means for preventing the gate electrode from beingaffected by the activation temperature in question, a damascene metalgate in which the metal noted above is buried attracts attention.However, since the work function of the metal electrode is positioned inthe vicinity of the mid gap of silicon, it is unavoidable to that theburied channel structure has poor short channel characteristics.

It should also be noted that, since a gate insulating film is formedagain after removal of the dummy gate once formed and the oxide filmpositioned below the dummy gate for burying an electrode material in thegate insulating film formed again, it is highly possible for a problemto be generated in terms of the reliability of the gate edge.

BRIEF SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provideda semiconductor device, comprising a semiconductor substrate; a pair offirst diffusion layers formed within the semiconductor substrate; a gateinsulating film formed on that portion of the semiconductor substratewhich is positioned between the paired diffusion layers; a gateelectrode including a first gate portion formed on the gate insulatingfilm and a second gate portion formed on the first gate portion, a firstwidth in a channel direction of the first gate portion beingsubstantially equal to a width in the channel direction of the gateinsulating film, and a second width in the channel direction of thesecond gate portion being larger than the first width; a gate side wallinsulating film including a first side wall portion formed on a sidesurface of the first gate portion and on a side surface of the gateinsulating film and a second side wall portion formed on a side surfaceof the second gate portion; and a second diffusion layer formed apartfrom the first diffusion layers within that portion of the semiconductorsubstrate which is positioned below the gate insulating film.

According to a second aspect of the present invention, there is provideda semiconductor device, comprising a semiconductor substrate; a pair offirst diffusion layers formed within the semiconductor substrate; a gateinsulating film including a first insulating film portion formed on thatportion of the semiconductor substrate which is positioned between thefirst diffusion layers and a second insulating film portion positionedon both edges of the first insulating film portion, a thickness of thesecond insulating film portion being larger than a thickness of thefirst insulating film portion; a gate electrode formed on the gateinsulating film; a gate side wall insulating film formed on a sidesurface of the gate electrode and on a side surface of the secondinsulating film portion; and a second diffusion layer formed apart fromthe first diffusion layers within that portion of the semiconductorsubstrate which is positioned below the first insulating film portion.

According to a third aspect of the present invention, there is provideda method of manufacturing a semiconductor device, comprising forming afirst material layer on a semiconductor substrate; forming a secondmaterial layer comprising a first width on the first material layer;partly removing the first material layer to leave the first materiallayer comprising a second width smaller than the first width below thesecond material layer; introducing an impurity into the semiconductorsubstrate with the second material layer used as a mask to form anextension region; forming a gate side wall insulating film on a sidesurfaces of the first and second material layers, the gate side wallinsulating film including a first side wall portion formed on the sidesurface of the first material layer and a second side wall portionformed on the side wall of the second material layer; introducing animpurity into the semiconductor substrate with the gate side wallinsulating film and the second material layer used as a mask to formsource and drain regions; forming an interlayer insulating film on thesemiconductor substrate, on the second material layer and on the gateside wall insulating film, followed by removing the interlayerinsulating film until the second material layer is exposed; removing thefirst and second material layers to form a groove; introducing animpurity through the groove into the semiconductor substrate to form asecond diffusion layer apart from the extension region within thatportion of the semiconductor substrate which is positioned below thegroove; forming a gate insulating film on that portion of thesemiconductor substrate which is positioned within the groove; andforming a gate electrode on the gate insulating film positioned withinthe groove.

According to a fourth aspect of the present invention, there is provideda method of manufacturing a semiconductor device, comprising forming agate insulating film on a semiconductor substrate; forming a secondmaterial layer comprising a predetermined shape on the gate insulatingfilm; thermally oxidizing the second material layer and thesemiconductor substrate to form a first insulating film on an uppersurface and a side surface of the second material layer and to increasea thickness in a portion of the gate insulating film; partly removingthe first insulating film and the gate insulating film to form a firstgate side wall insulating film on the side surface of the secondmaterial layer and to form a second insulating film portion of the gateinsulating film positioned below both edges of the second material layercomprising a thickness larger than a thickness of the first insulatingfilm portion of the gate insulating film below a central portion of thesecond material layer; introducing an impurity into the semiconductorsubstrate with the second material layer and the first gate side wallinsulating film used as a mask to form an extension region; forming asecond gate side wall insulating film on a side surface of the firstgate side wall insulating film; introducing an impurity into thesemiconductor substrate with the second material layer and the first andsecond gate side wall insulating films used as a mask to formsource-drain regions; forming an interlayer insulating film on thesemiconductor substrate, the second material layer and the first andsecond gate side wall insulating films, followed by removing theinterlayer insulating film until the second material layer is exposed;removing the second material layer to form a groove; introducing animpurity through the groove into the semiconductor substrate to form asecond diffusion layer apart from the extension region within thatportion of the semiconductor substrate which is positioned below thefirst insulating film portion; and forming a gate electrode on the gateinsulating film positioned within the groove.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a cross sectional view showing the construction of asemiconductor device according to a first embodiment of the presentinvention;

FIGS. 2A, 2B, 2C, 2D, 3A, 3B, 4A, 4B, 4C, 5A and 5B are cross sectionalviews collectively showing a method of manufacturing a semiconductordevice according to the first embodiment of the present invention;

FIG. 6 is a cross sectional view showing the construction of asemiconductor device according to a second embodiment of the presentinvention;

FIGS. 7A, 7B, 7C, 8A, 8B, 8C, 9A, 9B, 10A and 10B are cross sectionalviews collectively showing a method of manufacturing a semiconductordevice according to the second embodiment of the present invention;

FIG. 11A is a cross sectional view showing the construction of aconventional semiconductor substrate, which is used for describing thefunction and effect produced by the semiconductor device according toeach of the first and second embodiments of the present invention;

FIG. 11B is a cross sectional view showing the construction of asemiconductor substrate used in each of the first and second embodimentsof the present invention, which is used for describing the function andeffect produced by the semiconductor device according to each of thefirst and second embodiments of the present invention;

FIG. 12A is a cross sectional view showing the construction of aconventional semiconductor substrate, which is used for describing thefunction and effect produced by the semiconductor device according toeach of the first and second embodiments of the present invention;

FIG. 12B is a cross sectional view showing the construction of asemiconductor substrate used in each of the first and second embodimentsof the present invention, which is used for describing the function andeffect produced by the semiconductor device according to each of thefirst and second embodiments of the present invention;

FIGS. 13A, 13B and 13C are cross sectional views showing theconstruction of a semiconductor substrate used in each of the prior artand the first and second embodiments of the present invention, which areused for describing the function and effect produced by thesemiconductor device according to each of the first and secondembodiments of the present invention;

FIG. 14 is a cross sectional view showing the construction of asemiconductor device according to a third embodiment of the presentinvention;

FIGS. 15A, 15B, 15C, 16A, 16B, 16C, 17A, 17B, 17C, 18A and 18B are crosssectional views collectively showing a method of manufacturing asemiconductor device according to the third embodiment of the presentinvention; and

FIGS. 19A, 19B and 19C are cross sectional views each showing theconstruction of a semiconductor substrate, which is used for describingthe construction of a conventional semiconductor device.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention is directed to a damascene gatetransistor in which a gate electrode groove is formed in an insulatingfilm and a gate electrode is buried in the gate electrode groove thusformed. In embodiments of the present invention, the surface of asilicon substrate in which is formed a channel is oxidized so as to forma dummy gate made of, for example, polysilicon, followed by subjectingthe silicon oxide film right under the dummy gate to a side etching.Then, a side wall insulating film made of, for example, silicon nitride(SiN) is buried in the groove formed by the side etching so as to form aprojecting portion of the side wall insulating film projecting into aregion below the gate electrode. Further, the dummy gate and the oxidefilm below the dummy gate are removed in a manner to leave the side wallinsulating film unremoved, followed by performing an impurity doping foradjusting the threshold value. As a result, it is possible to improvethe short channel characteristics such that punch-through is unlikely totake place even if the channel length is decreased, thereby ensuring thereliability of the gate edge.

It should also be noted that an impurity doping for forming apunch-through stopper region is carried out together with the impuritydoping for adjusting the threshold value in a self-aligned fashion in aregion right under the extension region. As a result, it is possible tosuppress the elevation of the threshold value caused by the channelstopper while effectively suppressing the short channel effect.

Embodiments of the present invention will now be described withreference to the accompanying drawings. In the following description,like members or portions are denoted by like reference numeralsthroughout all the drawings.

[First Embodiment]

A first embodiment of the present invention will now be described withreference to FIGS. 1, 2A to 2D, 3A, 3B, 4A to 4C, 5A and 5B. FIG. 1 is across sectional view showing the construction of a semiconductor deviceaccording to the first embodiment of the present invention. On the otherhand, FIGS. 2A to 2D, 3A, 3B, 4A to 4C, 5A and 5B are cross sectionalviews collectively showing a method of manufacturing a semiconductordevice according to the first embodiment of the present invention.

The construction of the semiconductor device according to the firstembodiment of the present invention will now be described with referenceto FIG. 1.

As shown in the drawing, a semiconductor substrate 1 made of, forexample, silicon is divided by an element isolating region 2 such as anSTI (Shallow Trench Isolation) so as to form element regions. Thesemiconductor substrate 1 is, for example, P-type. In the firstembodiment of the present invention, a MOSFET of, for example, an N-typeis formed in the element region. Also, source-drain regions 3 of, forexample, an N-type are formed apart from each other in a surface regionof the element region formed in the semiconductor substrate 1. Alsoformed are extension regions 6 of, for example, N-type in those portionsof these source-drain regions 3 which are positioned to face each other.The structure including these N-type source-drain regions 3 and theN-type extension region 6 is called aherein later an SDE region(Source-Drain-Extension). A gate insulating film 5 made of, for example,a thermal oxide film of silicon is formed between the source and drainregions 3 in a manner to cover the surface of the semiconductorsubstrate 1. Also, a buried channel region 4 that is not in contact withthe extension regions 6 is formed below the gate insulating film 5 so asto be positioned between the extension regions 6.

Also, a gate electrode 7 made of a metal such as W, Al, Cu, or TiN(titanium nitride) or an alloy is formed on the gate insulating film 5.Side wall insulating films 8 each formed of, for example, a siliconnitride film are formed on both side surfaces of the gate electrode 7.The width of the gate electrode 7, which is called a gate length when itcomes to the construction of a transistor, is made larger than the widthof the gate insulating film 5. In other words, the gate insulating film5 is arranged substantially in the center of the gate electrode 7. Whenthe gate insulating film 5 overlaps the gate electrode 7, a clearance inwhich the gate insulating film is not present is formed between the gateelectrode 7 and the surface of the semiconductor substrate 1, and aprojecting portion 8′ of the side wall insulating film 8 is loaded inthe clearance noted above. To be more specific, the side wall insulatingfilm 8 projects into the clearance in question so as to form theprojecting portion 8′. The thickness of the projecting portion 8′ islarger than that of the gate insulating film 5. Both side regions on thebottom surface of the gate electrode 7 are formed on the projectingportion 8′. The projecting portion 8′ is generally called a terrace typespacer.

Also, an interlayer insulating film 9 formed of, for example, a siliconoxide film is formed by, for example, a CVD method on the surface of thesemiconductor substrate 1. The interlayer insulating film 9 is formed toexpose the surface of the gate electrode 7 and to bury the periphery ofthe gate electrode 7. Further, an interlayer insulating film 10 such asa silicon oxide film is formed on the interlayer insulating film 9 andon the exposed surface of the gate electrode 7. Connection wirings 11and 11′ electrically connected to the source-drain regions 3 are formedto extend through the interlayer insulating films 9 and 10. Each ofthese connection wirings 11 and 11′ is formed of, for example, tungsten.The connecting portions of these connection wirings 11, 11′ are exposedto the outside from the surface of the interlayer insulating film 10.

The semiconductor device according to the first embodiment of thepresent invention comprises the projecting portion 8′ of the side wallinsulating film 8, which projects into the inner region relative to theside surface of the gate electrode 7, with the result that a region thatis unlikely to be inverted is formed between the buried channel region 4and the extension region 6. It follows that it is possible to suppresspunch-through even if the channel length is decreased. In other words,the first embodiment of the present invention makes it possible toimprove the short channel characteristics so as to ensure thereliability of the gate edge.

A method of manufacturing the semiconductor device according to thefirst embodiment of the present invention will now be described withreference to FIGS. 2A to 2D, 3A, 3B, 4A to 4C, 5A and 5B.

In the first step, the element separating region 2 consisting of asilicon oxide film is formed on the semiconductor substrate 1, followedby forming an insulating film 12 such as a silicon oxide film, as shownin FIG. 2A. It suffices for the thickness of the insulating film 12,which determines the height of the terrace type spacer (projectingportion) 8′ described herein later, to be larger than that of the gateinsulating film 5 and to be large enough to ensure sufficiently thedifference in depth of the impurities doped by ion implantation. It isdesirable for the insulating film 12 to be formed of a silicon thermaloxide film having a thickness falling within a range of, for example,between 5 nm and 30 nm. It is also possible for the insulating film 12to be formed of a material that permits ensuring a suitable etchingselectivity ratio between the material for forming a gate dummy pattern14 referred to herein later and the semiconductor substrate in forming aside etching groove 15 in the subsequent step. For example, it ispossible to use a silicon nitride (SiN) film for forming the insulatingfilm 12.

In the next step, a material for forming the gate dummy pattern 14,e.g., a polysilicon film 13, is deposited in a thickness of about 200 nmon the element separating region 2 and the insulating film 12, as shownin FIG. 2B. It is possible for the polysilicon film 13 to be replaced bya film of a material that permits ensuring a suitable etchingselectivity ratio relative to the material of the neighboring member inthe subsequent step of forming a groove 16 for burying the gate.

Then, the polysilicon film 13 is selectively removed by, for example, aphotoresist process or a RIE (Reactive Ion Etching) process so as toform the gate dummy pattern 14, as shown in FIG. 2C. In the step ofremoving the polysilicon film 13, it is possible to etch partly orentirely the insulating film 12.

In the next step, the insulating film 12 is etched so as to form a sideetching groove 15 right under the gate dummy pattern 14, as shown inFIG. 2D. Where the insulating film 12 is formed of silicon oxide, anisotropic etching which permits etching in the lateral direction, too,is performed by using an oxide film etching solution such as ahydrofluoric acid. It is possible to easily control the length in thelateral direction of the side etching groove 15 by adjusting the etchingtime of the insulating film 12.

After formation of the side etching groove 15, an impurity for formingthe extension region 6 such as phosphorus or arsenic (in the case of anN-type MOSFET) is introduced into the surface region of thesemiconductor substrate 1 by means of ion implantation, so as to formthe extension regions 6, as shown in FIG. 3A. In this case, it ispossible to form in advance an oxide film having a thickness not solarge as to bury the side etching groove 15 as a screening oxide filmfor ion implantation. For example, where the side etching groove 15 hasa height of 10 nm, it is possible to form a screening oxide film havinga thickness of about 2 nm.

After ion implantation step, the side wall insulating films 8 are formedon the side surfaces of the gate dummy pattern 14 and within the sideetching groove 15, as shown in FIG. 3B. It is appropriate to use a filmcapable of filling the side etching groove 15, e.g., a silicon nitridefilm (SiN film) formed by an LPCVD method, which is satisfactory in thestep coverage, as the side wall insulating film 8. To be more specific,it is possible form a silicon nitride film or the like on the entiresurface, followed by etching back the silicon nitride film by RIE(Reactive Ion Etching) so as to form the side wall insulating film 8 tothe side of the gate dummy pattern 14. In this step, the silicon nitridefilm enters the side etching groove 15 so as to form the projectingportion 8′, or a terrace type spacer, of the side wall insulating film8.

In the next step, an impurity such as phosphorus or arsenic (in the caseof an N-type MOSFET) is introduced by means of ion implantation into thesemiconductor substrate 1 with the gate dummy pattern 14 and the sidewall insulating film 8 used as a mask, followed by performing activationunder high temperatures so as to form the source-drain regions 3, asshown in FIG. 4A.

After formation of the source-drain regions 3, an interlayer insulatingfilm 9 such as a silicon oxide film is deposited on the entire surface,followed by planarizing the surface of the interlayer insulating film 9by, for example, CMP (Chemical Mechanical Polishing) so as to expose thesurface of the gate dummy pattern 14 to the outside, as shown in FIG.4B.

In the next step, the gate dummy pattern 14 is selectively removed so asto form a groove 16 for burying a gate, as shown in FIG. 4C. Where thegate dummy pattern 14 is formed of polysilicon, it is possible to removeselectively the gate dummy pattern 14 by CDE (Chemical Dry Etching)using an etching gas such as CF₄ or by using a mixed acid consisting ofhydrofluoric acid and HNO₃. Incidentally, in the case of using the mixedacid, it is possible to control appropriately the selectivity ratio ofthe oxide film and polysilicon by controlling the ratio of hydrofluoricacid. Then, the insulating film 12 formed first is removed, followed byforming the terrace type spacer 8′. In this case, it is possible toremove the insulating film 12 by the treatment with hydrofluoric acid,if the side wall insulating film 8 is formed of a silicon nitride film.

In the next step, an impurity for forming the channel region such asphosphorus or arsenic (in the case of an N-type MOSFET) is introduced bymeans of ion implantation into the surface region of the semiconductorsubstrate 1 so as to form a buried channel region 4, as shown in FIG.5A. In this step, it is possible to dope the impurity in only thesemiconductor substrate 1 alone present in a specified region within thegroove 16 for burying the gate by controlling the accelerating energyfor ion implantation. For example, in the case of forming the buriedchannel region 4 of a damascene metal gate, the impurity for thepunch-through stopper is introduced deep and the impurity for formingthe buried channel region is introduced shallow by ion implantation.

In the next step, a gate insulating film 5 and a gate electrode 7 areformed within the groove 16 for burying the gate, as shown in FIG. 5B.In the case of using the gate insulating film 5 formed of a thermaloxide film, the gate oxide film 5 is formed shallower than the terracetype spacer 8′. For example, the gate insulating film 5 is formed to athickness suitable for maintaining the step of the terrace type spacer8′. Also, in the case of a damascene metal gate structure, a laminatestructure consisting of a TiN layer and a W layer is formed, followed byplanarizing the surface of the laminate structure by, for example, CMPso as to form the gate electrode 7. It is also possible to perform thedoping of an impurity by using polysilicon as a material of the gateelectrode.

Finally, after formation of the gate electrode 7, an insulating film 10is superposed on the insulating film 9, followed by forming contactholes extending through the insulating films 10 and 9 so as to form theconnection wires 11, 11′ connected to the source-drain regions 3, asshown in FIG. 1.

In the method of manufacturing a semiconductor device according to thefirst embodiment of the present invention, the side wall insulating film8 such as a silicon nitride film is buried in the sided etching groove15 so as to form the projecting portion 8′ of the side wall insulatingfilm 8 extending into a region below the gate dummy pattern 14, followedby removing the gate dummy pattern 14 and the oxide film 12 positionedbelow the gate dummy pattern 14. Further, the side wall insulating film8 is left unremoved, and the impurity doping for controlling thethreshold value is performed. As a result, it is possible to improve theshort channel characteristics such that the punch-through is rendereddifficult even if the channel length is decreased so as to ensure thereliability of the gate edge.

[Second Embodiment]

A second embodiment of the present invention will now be described withreference to FIGS. 6, 7A to 7C, 8A to 8C, 9A, 9B, 10A and 10B. FIG. 6 isa cross sectional view showing the construction of the semiconductordevice according to the second embodiment of the present invention. Onthe other hand, FIGS. 7A to 7C, 8A to 8C, 9A, 9B, 10A and 10B are crosssectional views collectively showing the method of manufacturing thesemiconductor device according to the second embodiment of the presentinvention.

First of all, the construction of the semiconductor device according tothe second embodiment of the present invention will now be describedwith reference to FIG. 6.

As shown in the drawing, an element region separated by an elementseparating region 102 such as an STI is formed in, for example, a P-typesemiconductor substrate 201 consisting of, for example, silicon. N-typesource-drain regions 203 having N-type extension regions 206 formed inthose portions of the source-drain regions 203 which are positioned toface each other (SDE region) are formed apart from each other in thesurface region of the element region included in the semiconductorsubstrate 201. A gate insulating film 205 consisting of, for example, athermal oxide film of silicon is formed to cover the surface of thatportion of the semiconductor substrate 201 which is positioned betweenthe source-drain regions 203. Also, a buried channel region 204, whichis not in contact with the extension regions 206, is formed between theextension regions 206 below the gate insulating film 205.

Also, a gate electrode 207 made of a metal such as W, Al, Cu or TiN oran alloy is formed on the gate insulating film 205. Side wall insulatingfilms 208 are formed on both side surfaces of the gate electrode 207.The side wall insulating film 208 includes a first portion 215 incontact with the gate electrode 207, a second portion 217 covering theouter surface of the first portion 215, and a projecting portion 208′positioned right under the gate electrode 207 and formed in the bothside regions of the gate insulating film 205. The projecting portion208′, which is formed by heating the gate insulating film 205, is formedthicker than the gate insulating film 205.

The width (gate length) of the gate electrode 207 is larger than thewidth of the gate insulating film 205 excluding the projecting portion208′. To be more specific, the gate insulating film 205 is arrangedsubstantially in the center of the gate electrode 207 such that, whenthe gate insulating film 205 overlaps with the gate electrode 207, theprojecting portions 208′ are formed between the both side portions ofthe gate electrode 207 and the surface of the semiconductor substrate201. In other words, both side regions on the bottom surface of the gateelectrode 207 are formed on the projecting portion 208′. The projectingportion 208′ is generally called a terrace type spacer.

An interlayer insulating film 209 such as a silicon oxide film is formedby, for example, a CVD method on the semiconductor substrate 201. Theinterlayer insulating film 209 is formed to expose the surface of thegate electrode 207 to the outside and to bury the periphery of the gateelectrode 207. Further, an interlayer insulating film 210 such as asilicon oxide film is formed to cover the surface of the interlayerinsulating film 209 and the exposed surface of the gate electrode 207.It should be noted that connection wirings 211, 211′ electricallyconnected to the source-drain regions 203 are formed to extend throughthe interlayer insulating films 209 and 210. Each of these connectionwirings 211 and 211′ is formed of, for example, tungsten, and theconnecting portions of these connection wirings 211, 211′ are exposed tothe outside from the surface of the interlayer insulating film 210.

The semiconductor device according to the second embodiment of thepresent invention includes the first portion 215 of the side wallinsulating film 208 formed on the side surface of the gate electrode 207and the projecting portion 208′ formed on both side regions of the gateinsulating film 205 so as to form a region that is unlikely to beinverted between the buried channel region 204 and the extension region206. As a result, punch-through is rendered difficult even if thechannel length is decreased. It follows that the second embodiment ofthe present invention makes it possible to improve the short channelcharacteristics so as to ensure the reliability of the gate edge.

A method of manufacturing the semiconductor device according to thesecond embodiment of the present invention will now be described withreference to FIGS. 7A to 7C, 8A to 8C, 9A, 9B, 10A and 10B.

In the first step, an element separating region 202 such as an STI isformed on the semiconductor substrate 201, followed by forming a gateinsulating film 205 such as a silicon oxide film in the element region,as shown in FIG. 7A. Then, the material of the gate dummy pattern 214,such as polysilicon is deposited to a thickness of about 200 nm to formthe film 213 on the gate insulating film 205 and the element separatingregion 202.

In the next step, the polysilicon film 213 is patterned through aphotoresist forming step, RIE step, etc. so as to form a gate dummypattern 214, as shown in FIG. 7B. Further, the surfaces of thesemiconductor substrate 201 and the gate dummy pattern 214 are thermallyoxidized so as to form a post-oxide film 215 on the surface of the gatedummy pattern 214, as shown in FIG. 7C. In this step, a bird's beakregion is formed in the edge portion on the bottom surface of the gatedummy pattern 214 so as to form a terrace type spacer (projectingportion) 208.

In the next step, RIE is applied to the post-oxide film 215 so as tohave the side surface of the gate dummy pattern 214 and the portion ofthe terrace type spacer 208′ left unremoved, as shown in FIG. 8A. Then,an N-type impurity such as phosphorus or arsenic is introduced in thecase of an N-type MOSFET into the semiconductor substrate 201 by meansof ion implantation with the gate dummy pattern 214 used as a mask so asto form extension regions 206.

Further, an insulating film 217 such as a silicon nitride film (SiNfilm) is formed on the side surface of the post-oxide film 215 in thegate dummy pattern 214. As a result, formed is a side wall insulatingfilm 208 consisting of the post-oxide film 215, the insulating film 217and the projecting portion 208′. The insulating film 217 is formed asfollows. Specifically, a silicon nitride film or the like is formed onthe entire surface of the substrate, followed by etching back thesilicon nitride film by RIE so as to form the side wall structure to theside of the post-oxide film 215.

In the next step, an N-type impurity such as phosphorus or arsenic isintroduced by means of ion implantation in the case of an N-type MOSFETwith the gate dummy pattern 214 and the side wall insulating film 208used as a mask, followed by applying an activation treatment under hightemperatures so as to form source-drain regions 203 connected to theextension regions 206, as shown in FIG. 8C. After formation of thesource-drain region 203, an interlayer insulating film 209 such as asilicon oxide film is deposited, followed by planarizing the surface ofthe interlayer insulating film by, for example, CMP so as to expose thesurface of the gate dummy pattern 214, as shown in FIG. 9A.

Then, the gate dummy pattern 214 is selectively removed by etching so asto form a groove 216 for burying a gate, as shown in FIG. 9B. Further,an N-type impurity such as arsenic or phosphorus is introduced by meansof ion implantation so as to form a buried channel region 204, as shownin FIG. 10A. In this case, it is possible to dope the impurity in only aspecified region of the semiconductor substrate 201 within the groove216 for burying the gate by controlling the accelerating energy in ionimplantation step.

Further, a gate electrode 207 is formed on the gate insulating film 205within the groove 216 for burying the gate, as shown in FIG. 10B. Inthis case, it is possible to use the oxide film 205 formed below thegate dummy pattern 214 as the gate insulating film 205 as in this secondembodiment. It is also possible to form again the gate insulating film205 by once peeling off the screening oxide film used in ionimplantation step. In the case of forming again the gate insulating film205, the damage or the influence of contamination or the like areunlikely to remain. Also, in the case of a damascene metal gatestructure, the gate electrode 207 is formed by forming a laminatestructure consisting of a TiN layer and a W layer, followed byplanarizing the surface of the laminate structure by, for example, CMP.It is also possible to perform an impurity doping by the same process byusing polysilicon as a material of the gate electrode.

Finally, an insulating film 210 is superposed on the insulating film209, followed by forming the connection wirings 211 and 211′ connectedto the source-drain regions 203 by forming contact holes extendingthrough the insulating films 209 and 210 so as to obtain a desiredtransistor as shown in FIG. 6.

According to the second embodiment of the present invention describedabove, the thermal oxide film 215 is formed on the side surface of thegate electrode 207, and the projecting portions 208′ are formed on bothside regions of the gate insulating film 205. As a result, formed is aregion that is unlikely to be inverted between the buried channel region204 and the extension region 206. It follows that punch-through isunlikely to take place even if the channel length is decreased. In thisfashion, the second embodiment of the present invention makes itpossible to improve the short channel characteristics so as to ensurethe reliability of the gate edge.

Further, in the case where the gate insulating film 205 is formed of anoxide film, it is possible to increase appropriately the thickness ofthe edge portion (projecting portion 208′) of the gate insulating film205 so as to make it possible to expect improvements in the gatebreakdown voltage and the reliability.

FIGS. 11 to 13 are cross sectional views of semiconductor substratesused in the prior art and the first and second embodiments of thepresent invention for describing the functions and effects produced bythe first and second embodiments of the present invention. The functionsand effects of the present invention will now be described in comparisonwith the prior art with the semiconductor device according to the firstembodiment of the present invention taken as an example.

In the semiconductor device of the conventional structure shown in FIG.11A, the counter ion implantation region (buried channel region 4) is incontact with the extension region 6. In other words, the impurityregions of the same conductivity type are joined to each other, with theresult that punch-through tends to take place easily.

On the other hand, in the structure according to the first embodiment ofthe present invention, the thick terrace type spacer 8′ (projectingportion of the side wall insulating film) is present in both edgeportions of the groove 16 for performing channel ion implantation asshown in FIG. 11B (corresponding to the semiconductor device shown inFIG. 1). It follows that it is possible to form the counter ionimplantation region (buried channel region 4), apart from the extensionregion 6. Because of this particular construction, a region that isunlikely to be inverted is formed between the buried channel region 4and the extension region 6, with the result that punch-through isunlikely to take place even if the channel length is decreased.

Also, in the semiconductor device of the conventional structure as shownin FIG. 12A, an overlapping capacitance C1 is increased in theoverlapping portion between the diffusion portion of the extensionregion 6 in the lateral direction into a region below the gate electrode7 and the gate electrode 7.

On the other hand, in the construction according to the first embodimentof the present invention, it is possible to form thick the insulatingfilm (projecting portion 8′) in the overlapping portion of the gateelectrode 7, as shown in FIG. 12B, so as to make it possible to decreasethe overlapping capacitance C2, thereby improving the transistorcharacteristics.

It should also be noted that it is possible to control optionally theheight and the length in the lateral direction of the terrace typespacer (projecting portion of the side wall insulating film) 8′ in bothedges of the region where a channel is to be formed by employing theprocess according to the first embodiment of the present invention inthe process of forming a damascene gate transistor. In other words, itis possible to control the height and the length in the lateraldirection of the terrace type spacer (projecting portion) 8′ bycontrolling the thickness of the insulating film 12, such as the siliconoxide film shown in FIG. 2A, and by controlling the size of the sideetching groove 15 shown in FIG. 2D through adjustment of the etchingamount. As a result, a difference in the formation of the impurityregion is generated between the case where the terrace type spacer 8′ isthin and the case where the terrace type spacer 8′ is thick even if ionimplantation of the same energy is carried out. To be more specific,where the terrace type spacer 8′ is thin, the impurity is doped in thesemiconductor substrate 1 below the terrace type spacer 8′, as shown inFIG. 13A. On the other hand, where the terrace type spacer 8′ is thick,an impurity 17 is shielded by the terrace type spacer 8′ so as toprevent the impurity 17 from being doped in the semiconductor substrate1 below the terrace type spacer 8′, as shown in FIG. 13B.

Also, in the case of forming the punch-through stopper region, thethickness of the terrace type spacer (projecting portion) 8′ is set at,for example, 10 nm in the step of introducing a channel impurity 17′ forthe buried channel region of an n-MOS transistor by means of ionimplantation. At this time, a stopper impurity (e.g., boron) 17 forforming the punch-through stopper region is introduced by means of ionimplantation under an accelerating energy of 20 KeV. In this case, thestopper impurity 17 is formed shallow below the terrace type spacer 8′and is formed deep below the channel region on which the terrace typespacer 8′ is not formed, as shown in FIG. 13C.

Also, in the case of introducing a channel impurity (e.g., arsenic As)17′ for forming a buried channel region by means of ion implantationunder an accelerating energy of 5 KeV, the impurity 17′ does not enterthe semiconductor substrate 1 below the terrace type spacer 8′, and theimpurity distribution including the impurity 17′ is formed in thechannel region on which the terrace type spacer 8′ is not formed, asshown in FIG. 13C. In the case of the particular construction (i.e., inthe construction including the buried channel region 4 shown in FIG. 1),it is possible to realize a construction in which the impurityconcentration of the conductivity type equal to that in thesemiconductor substrate 1 is high and the buried region concentration islow in the edge portion on the surface of the channel region and thatthe impurity concentration of the conductivity type equal to that in thesemiconductor substrate 1 is low and the buried region concentration ishigh in the central portion on the surface of the channel. In otherwords, it is possible to produce an effect similar to that produced by aHalo structure without increasing the parasitic capacitance between theextension region 6 and the semiconductor substrate or between thesource-drain regions 3 and the semiconductor substrate.

As described above, it is possible to control the distance between theextension region 6 and the buried channel region 4 by adjusting thelength of the terrace type spacer 8′. In other words, in the secondembodiment of the present invention, it is possible to control thedegree of punch-through in conformity with the impurity concentrationand distribution in the channel and the desired channel length so as toincrease the degree of freedom for optimizing the elementcharacteristics.

Also, it is possible to improve the controllability in terms of theoverlapping capacitance between the gate electrode 7 and the extensionregion 6 or between the gate electrode 7 and the source-drain regions 3,the junction capacitance between the channel region and the extensionregion 6 or between the channel region and the source-drain regions, theeffective gate length, and the channel profile in the lateral directionso as to increase the degree of freedom in the design of the channel.

[Third Embodiment]

A third embodiment of the present invention will now be described withreference to FIGS. 14, 15A to 15C, 16A to 16C, 17A to 17C, 18A and 18B.FIG. 14 is a cross sectional view showing the construction of thesemiconductor device according to the third embodiment of the presentinvention. On the other hand, FIGS. 15A to 15C, 16A to 16C, 17A to 17C,18A and 18B are cross sectional views collectively showing the method ofmanufacturing the semiconductor device according to the third embodimentof the present invention.

First of all, the construction of the semiconductor device according tothe third embodiment of the present invention will now be described withreference to FIG. 14.

As shown in the drawing, an element region separated by an elementseparating region 402 such as an STI is formed in, for example, a P-typesemiconductor substrate 401 consisting of, for example, silicon. N-typesource-drain regions 403 having N-type extension regions 406 formed inthose portions of the source-drain regions 403 which are positioned toface each other (SDE region) are formed apart from each other in thesurface region of the element region included in the semiconductorsubstrate 401. A gate insulating film 405 consisting of, for example, athermal oxide film of silicon is formed to cover the surface of thatportion of the semiconductor substrate 401 which is positioned betweenthe source-drain regions 403.

Also, a gate electrode 407 made of a metal such as W, Al, Cu or TiN oran alloy is formed on the gate insulating film 405. Side wall insulatingfilms 408 are formed on both surfaces of the gate electrode 407. Itshould be noted that the width (gate length) of the gate electrode 407is larger than the width of the gate insulating film 405. In otherwords, when the gate insulating film 405 is arranged substantially inthe center of the gate electrode 407 so as to allow the gate insulatingfilm 405 and the gate electrode 407 to overlap each other, a clearancein which the gate insulating film is not present is formed between thegate electrode 407 and the semiconductor substrate 401. A projectingportion 408′ projecting from the side wall insulating film 408 is loadedin the clearance noted above. The projecting portion 408′ is formedthicker than the gate insulating film. Further, both side edge regionson the bottom surface of the gate electrode 407 are positioned on theprojecting portion 408′. The projecting portion 408′ of this particularconstruction is called a terrace type spacer.

Also, a buried channel region 404 is formed below the gate insulatingfilm 405 between the extension regions 406. The buried channel region404 is not in contact with either the extension region 406 or the gateinsulating film 405. Also, a punch-through stopper region 404′ is formedbelow the buried channel region 404. It should be noted that, in thepunch-through stopper region 404′, the peak concentration in a firstportion 404′a, positioned below the projecting portion 408′, is lowerthan the peak concentration in a second portion 404′b positioned belowthe buried channel region 404.

Also, an interlayer insulating film 409 such as a silicon oxide film isformed on the semiconductor substrate 401 by, for example, a CVD method.The interlayer insulating film 409 exposes the surface of the gateelectrode 407 and buries the periphery of the gate electrode 407.Further, an interlayer insulating film 410 such as a silicon oxide filmis formed to cover the surface of the interlayer insulating film 409 andthe exposed surface of the gate electrode 407. Still further, connectionwirings 411 and 411′ electrically connected to the source-drain regions403 are formed to extend through the interlayer insulating films 409 and410. Each of these connection wirings 411 and 411′ is formed of, forexample, tungsten. Also, the connecting portions of these connectionwirings 411, 411′ are exposed to the surface of the interlayerinsulating film 410.

The semiconductor device according to the third embodiment of thepresent invention described above includes the projecting portion 408′of the side wall insulating film 408 projecting inward relative to theside surface of the gate electrode 407, with the result that a regionthat is unlikely to be inverted is formed between the buried channelregion 404 and the extension region 406. It follows that punch-throughis unlikely to take place even if the channel length is decreased. Inother words, the third embodiment of the present invention makes itpossible to improve the short channel characteristics so as to ensure areliability of the gate edge.

It should also be noted that the punch-through stopper region 404′ isformed in a self-aligned fashion right under the extension region 406 soas to make it possible to control the elevation of the threshold valueowing to the channel stopper while effectively suppressing the shortchannel effect.

A method of manufacturing a semiconductor device according to the thirdembodiment of the present invention will now be described with referenceto FIGS. 15A to 15C, 16A to 16C, 17A to 17C, 18A and 18B.

In the first step, an element separating region 402 such as an STI isformed on the semiconductor substrate 401, followed by forming aninsulating film 412 such as a silicon oxide film in the element region,as shown in FIG. 15A. It suffices for the thickness of the insulatingfilm 412, which determines the height of a terrace type spacer(projecting portion) 408′ referred to herein later, to be larger thanthat of the gate insulating film and to be large enough to ensuresufficiently the difference in the depth of the impurity doped in ionimplantation step. It is desirable for the insulating film 412 to beformed of a silicon thermal oxide film having a thickness of, forexample, between 5 nm and 30 nm. Also, it suffices for the insulatingfilm 412 to be capable of ensuring an etching selectivity ratio betweenthe material of a gate dummy pattern 414 referred to herein later andthe semiconductor substrate in the subsequent step of forming a sideetching groove 415. In this sense, it is possible to use, for example, asilicon nitride film (SiN film) as the insulating film 412.

In the next step, a material used for forming the gate dummy pattern414, e.g., a polysilicon film 413, is deposited to a thickness of about200 nm on the element separating region 402 and the insulating film 412,as shown in FIG. 15B. It is also possible to use, in place of thepolysilicon film 413, a film of a material capable of ensuring anetching selectivity ratio relative to the material of the neighboringmember in the subsequent step of forming a groove 416 for burying thegate.

In the next step, the polysilicon film 413 is selectively removed by,for example, a photoresist process or a RIE process so as to form thegate dummy pattern 414, as shown in FIG. 15C. In this step, it ispossible to remove partly or entirely the insulating film 412 by theetching simultaneously with the selective removal of the polysiliconfilm 413.

Then, the insulating film 412 is etched so as to form a side etchinggroove 415 right under the gate dummy pattern 414 as shown in FIG. 16A.Where the insulating film 412 is formed of silicon oxide, the insulatingfilm 412 is subjected to an isotropic etching, which also permits theetching in the lateral direction, using an oxide film etching solutionsuch as hydrofluoric acid. It is possible to control easily the lengthin the lateral direction of the side etching groove 415 by adjusting theetching time of the insulating film 412.

In the next step, an impurity such as phosphorus or arsenic for formingan extension region 406 (in the case of an N-type MOSFET) is introducedinto the semiconductor substrate 401 by means of ion implantation withthe gate dummy pattern 414 used as a mask so as to form extensionregions 406. In this case, it is possible to form in advance an oxidefilm having such a thickness as not to bury the side etching groove 415as a screening oxide film for ion implantation. For example, where theside etching groove 415 has a height of 10 nm, it is possible to form anoxide film having a thickness of about 2 nm.

In the next step, a side wall insulating film 408 is formed on each ofthe gate dummy pattern 414 and within the side etching groove 415 asshown in FIG. 16C. It is desirable for the side wall insulating film 408to be formed of a material that can be used for filling the side etchinggroove 415, e.g., a silicon nitride film (SiN film) formed by an LPCVDmethod, which is satisfactory in the step coverage. It suffices to formthe side wall insulating film 408 sideward of the gate dummy pattern 414by depositing, for example, a silicon nitride film on the entiresurface, followed by etching back the silicon nitride film thus formed.In this step, the silicon nitride film extends to enter the side etchinggroove 415 so as to form a projecting portion (terrace type spacer) 408′of the side wall insulating film 408.

Then, in the case of an N-type MOSFET, an impurity such as phosphorus orarsenic is introduced into the semiconductor substrate 401 by ionimplantation with the gate dummy pattern 414 and the side wallinsulating film 408 used as a mask, followed by performing an activatingtreatment under a high temperature so as to form source-drain regions403, as shown in FIG. 17A. After formation of the source-drain regions403, an interlayer insulating film 409 such as a silicon oxide film isdeposited, followed by planarizing the surface of the interlayerinsulating film 409 by, for example, CMP so as to expose the surface ofthe dummy gate pattern 414, as shown in FIG. 17B.

In the next step, the gate dummy pattern 414 is selectively removed soas to form a groove 416 for burying the groove, as shown in FIG. 17C.Where the gate dummy pattern 414 is made of polysilicon, it is possibleto remove selectively the gate dummy pattern 414 by CDE using an etchinggas such CF₄ or by using a mixed acid consisting of hydrofluoric acidand HNO₃. In the case of using the mixed acid, it is possible to controlappropriately the selectivity ratio between the oxide film and thepolysilicon film (gate dummy pattern 414) by adjusting the ratio of thehydrofluoric acid in the mixed acid. Then, the insulating film 412formed first is removed so as to form a terrace type spacer 408′. Inthis case, if the side wall insulating film 408 is formed of a siliconnitride film, it is possible to remove the insulating film 412 by thetreatment with hydrofluoric acid.

Then, in the case of an N-type MOSFET, an impurity for forming a channelregion such as phosphorus or arsenic is implanted into the semiconductorsubstrate 401 by means of ion implantation so as to form a buriedchannel region 404, as shown in FIG. 18A. In this step, it is possibleto dope the impurity into only a specified region of the semiconductorsubstrate 401 within the groove 416 for burying the gate by controllingthe accelerating energy for ion implantation. Also, a punch-throughstopper region 404′ is formed by introducing an impurity of theconductivity type equal to that of the impurity contained in thesemiconductor substrate 401 by means of ion implantation such that theparticular impurity is introduced deeper than the impurity for formingthe buried channel.

In the next step, a gate insulating film 405 and a gate electrode 407are formed within the groove 416 for burying the electrode, as shown inFIG. 18B. In the case of using the gate insulating film 405 consistingof a thermal oxide film prepared by oxidizing the semiconductorsubstrate 401, the gate insulating film 405 is formed thinner than theterrace type spacer 408′. For example, the gate insulating film 405 isformed to a thickness that permits maintaining the step with the terracetype spacer 408′. Also, in the case of a damascene metal gate structure,the gate electrode 407 is formed by forming first a laminate structureconsisting of a TiN layer and a W layer, followed by planarizing thesurface of the laminate structure by, for example, CMP. It is alsopossible to perform an impurity doping in the case of using polysiliconas a material of the gate electrode by using a similar process.

Finally, an insulating film 410 is formed on the insulating film 409,followed by forming contact holes extending through the insulating films410 and 409 so as to form connection wirings 411, 411′ connected to thesource-drain regions 403, as shown in FIG. 14.

In the third embodiment of the present invention described above, theside wall insulating film 408 such as a silicon nitride film is buriedin the side etching groove 415 so as to form the projecting portion 408′of the side wall insulating film 408 in a manner to extend into a regionbelow the gate dummy pattern 414. Then, after the gate dummy pattern 414and the oxide film 412 positioned below the gate dummy pattern 414 areremoved, the side wall insulating film 408 is allowed to remain,followed by performing the doping of the impurity for controlling thethreshold value. As a result, the short channel characteristics areimproved so as to allow punch-through to be unlikely to take place evenif the channel length is decreased, thereby ensuring the reliability ofthe gate edge.

It is also possible to suppress the elevation of the threshold valuecaused by the channel stopper while suppressing effectively the shortchannel effect by performing in a self-aligned fashion the impuritydoping for forming the punch-through stopper region 404′ right under theextension region 406 and the impurity doping for controlling thethreshold value.

It is also possible to control optionally the height and the length inthe lateral direction of the terrace type spacer (projecting portion ofthe side wall insulating film) 408′ at both edge portions of the regionwhere the channel is to be formed by employing the process according tothe third embodiment of the present invention in the process for forminga damascene gate transistor. As a result, it is possible to improve thecontrollability in terms of the overlapping capacitance between the gateelectrode 407 and the extension region 406 or between the gate electrode407 and the source-drain regions 403, the junction capacitance betweenthe channel region and the extension region 406 or between the channelregion and the source-drain regions 403, the effective gate length, andthe channel profile in the lateral direction so as to increase thedegree of freedom in the design of the channel.

It should be noted in particular that it is possible to form the veryshallow buried channel region 404 apart from the source-drain regions403 by an optional distance in the damascene metal gate process, inwhich the source-drain regions 403 are formed with the dummy gatepattern 414 once formed used as a mask, the dummy gate pattern 414 isremoved after activation of the source-drain regions, the gateinsulating film 405 is formed again after ion implantation into thechannel region and, then, the metal electrode material is buried. It isalso possible to form the impurity region used as the punch-throughstopper region 404′ together with the buried channel region 404 apartfrom the source-drain regions 403 in a self-aligned fashion such thatthe impurity region noted above is formed shallow in the edges of thesource-drain regions 403 and deep in the central portion of the channelregion. As a result, it is possible to form a transistor free fromdeterioration of the short channel characteristics in the buried channelregion 403′.

Further, in the case of forming a surface channel type transistorcomprising the gate electrode 407 formed of polysilicon or two kinds ofmetallic material differing from each other in the work function, it ispossible to form similarly the punch-through stopper region 404′ in aself-aligned fashion apart from the source-drain regions 403 such thatthe punch-through stopper region 404′ is shallow in the edge portions ofthe source-drain regions 403 and deep in the central portion of thechannel region.

Still further, in the case of both the buried channel type and thesurface channel type, it is possible to suppress the parasiticcapacitance between the gate electrode 407 and the source-drain regions403 and the parasitic capacitance between the source-drain regions 403and the semiconductor substrate 401.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the present invention in its broaderaspects is not limited to the specific details and representativeembodiments shown and described herein. Accordingly, variousmodifications may be made without departing from the spirit or scope ofthe general inventive concept as defined by the appended claims andtheir equivalents.

What is claimed is:
 1. A semiconductor device, comprising: asemiconductor substrate; a pair of first diffusion layers formed withinsaid semiconductor substrate; a gate insulating film formed on thatportion of said semiconductor substrate which is positioned between saidpaired diffusion layers; a gate electrode including a first gate portionformed on said gate insulating film and a second gate portion formed onsaid first gate portion, a first width in a channel direction of saidfirst gate portion being substantially equal to a width in said channeldirection of said gate insulating film, and a second width in saidchannel direction of said second gate portion being larger than saidfirst width; a gate side wall insulating film including a first sidewall portion formed on a side surface of said first gate portion and ona side surface of said gate insulating film and a second side wallportion formed on a side surface of said second gate portion; a seconddiffusion layer formed apart from said first diffusion layers withinthat portion of said semiconductor substrate which is positioned belowsaid gate insulating film; and a third diffusion layer including a firstdiffusion portion formed below said first diffusion layers under saidfirst side wall portion and a second diffusion portion formed below saidsecond diffusion layer, wherein a distance of a peak portion of aimpurity concentration in said first diffusion portion from a surface ofsaid semiconductor substrate is less than a distance of a peak portionof a impurity concentration in said second diffusion portion from thesurface of said semiconductor substrate, and wherein a conductivity typeof said third diffusion layer is equal to the conductivity type of saidsemiconductor substrate.
 2. The semiconductor device according to claim1, wherein said first diffusion layers further comprises: a pair ofextension regions formed below said second side wall portion apart fromsaid second diffusion layer; and a pair of source-drain regions formedin contact with said extension regions on a side opposite said seconddiffusion layer.
 3. The semiconductor device according to claim 1,further comprising an interlayer insulating film formed to surround saidgate side wall insulating film, an upper surface of said interlayerinsulating film being substantially equal to an upper surface of saidgate electrode.
 4. The semiconductor device according to claim 1,wherein a conductivity type of said second diffusion layer is oppositethe conductivity type of said semiconductor substrate.
 5. Thesemiconductor device according to claim 1, wherein said gate insulatingfilm is thicker than said first side wall portion.